The present invention relates to a developing device for a copier, facsimile transceiver, printer or similar electrophotographic image forming apparatus using a dry developer.
It is a common practice with a developing device for the above application to agitate a powdery developer, or toner, stored in a hopper by an agitator while conveying it to a developing roller. The agitator is mounted on one end of a rotary agitator shaft which protrudes to the outside of the hopper at the other end thereof. A motor is connected to the protruding end of the agitator shaft via a gearing to rotate the agitator shaft at a constant speed at all times, i.e., with no regard to the varying amount of toner in the hopper. This is not desirable for the following reasons. When a great amount of toner exists in the hopper, the agitator may be rotated at a relatively low speed since it is capable of conveying and supplying the toner to the developing roller efficiently. However, when the amount of toner remaining in the hopper is small, the rotation of the agitator has to be accelerated to compensate for the degration of conveying and supplying ability. It has been customary, however, to rotate the agitator at a constant speed, i.e., at a speed high enough to sufficiently convey the toner even when the toner remaining in the hopper is scarce. It follows that the agitator is undesirably rotated at a high speed when a great amount of toner exists in the hopper, supplying an excessive amount of toner to the developing roller. This brings about blocking, i.e., causes the developer to form blocks in the vicinity of the developing roller to thereby degrade the toner supply. Moreover, the toner fed in an excessive amount is apt to blow out through insufficiently sealed portions of the developing device. In addition, excessive stresses acting on the toner tend to deteriorate the toner.